1. Technical Field
The present disclosure relates to the field of integrated circuit fabrication. The present disclosure relates more particularly to the gettering of semiconductor wafers.
2. Description of the Related Art
Integrated circuits are typically fabricated in and on substrates of monocrystalline semiconductor material. Integrated circuits often include MOS or bipolar transistors formed in the monocrystalline semiconductor substrate. MOS transistors include source, drain, channel, and body regions formed in the monocrystalline semiconductor substrate. Bipolar transistors typically include emitter, collector, and base regions in the monocrystalline semiconductor substrate.
In order to promote reliable and efficient function of the transistors, it is desirable to remove unwanted impurity atoms from the monocrystalline semiconductor substrate. Impurity atoms, especially copper, iron, and gold atoms, in the monocrystalline semiconductor substrate can have adverse effects on the function of transistors. Impurity atoms can cause junction leakage in which leakage currents increase the power dissipation of the circuit and cause unwanted heating of the circuit. Impurity atoms can also create trap states in the bandgap of the monocrystalline semiconductor substrate. This means that electrons in the area of the impurity atom may become trapped in an energy state within the bandgap rather than in the valence or conduction bands outside of the bandgap. These energy trap states cause degradation in the function of the transistors which rely on the bandgap of the semiconductor atoms in the monocrystalline semiconductor substrate for proper function. If too many impurity atoms are in the substrate then individual transistors may perform poorly or not at all. Function of the entire integrated circuit may be jeopardized if too many transistors are affected.
Gettering is a process that removes impurity atoms from the monocrystalline semiconductor substrate. Even if gettering is very thorough during crystal growth, impurity atoms may enter the substrate in subsequent processing steps. Gettering may therefore be performed during and after subsequent processing steps to remove impurities from the bulk regions of the monocrystalline semiconductor substrate in which the transistors will be formed.
Processes for gettering semiconductor substrates are generally divided into two categories: Extrinsic gettering and intrinsic gettering. In extrinsic gettering external influences are applied to introduce stress in the monocrystalline semiconductor lattice. The stressed areas act as traps for the impurity atoms.
Extrinsic gettering can be performed by applying stress or damage to the backside of the semiconductor substrate. This can be accomplished by sandblasting the backside of the wafer, scoring the backside of the wafer, irradiating the backside of the wafer with a high intensity laser beam, or bombarding the backside of the wafer with high energy ions. Each of these methods can introduce stressed areas in the semiconductor lattice that will act as traps for the impurity atoms and thus perform gettering.
Intrinsic gettering methods include precipitating saturated oxygen out of the wafer and creating stressed areas in the semiconductor lattice that eventually disrupt areas of the lattice and create trapping areas for impurities. This means that a wafer should have some amount of oxygen put in during formation of the semiconductor lattice. This can be an improvement over extrinsic gettering because no further wafer processing except for heating is needed to perform the gettering. Additionally the gettering sites can be larger than with some extrinsic methods and the gettering sites are closer to the bulk regions. However, these typical extrinsic and intrinsic gettering methods can be difficult or impossible to perform in silicon-on-insulator (SOI) wafers.
Gettering may be particularly difficult in SOI chips in which an insulating substrate is attached to the backside of the bulk monocrystalline semiconductor substrate. Having an insulating substrate coupled to the back side of the wafer greatly increases the difficulty of gettering of the monocrystalline semiconductor substrate.